With the spread of the Internet, development of large capacity optical communication systems has been promoted. In a trunk system, for example, research has been conducted on an optical transmitter and an optical receiver that are capable of transmitting signals over 40 Gbit/s per wavelength. Further, there has been an increase in research and development on a polarization multiplexing transmission system to improve spectrum use efficiency of wavelength multiplexing transmission.
In a polarization multiplexing optical transmission system, a set of data signals are transmitted by utilizing X polarization and Y polarization (H/V polarizations). The X polarization and the Y polarization are a pair of orthogonal polarizations. In an optical receiver, a set of data signals are extracted by separating the X polarization and the Y polarization from each other. Therefore, the polarization multiplexing transmission system enables the capacity per wavelength to double.
A polarization multiplexing optical receiver generally has a polarization control function to control a polarization state of an input optical signal and has a separating function to obtain orthogonal polarizations (X polarization channel and Y polarization channel) of the polarization-controlled optical signal. The X polarization channel and the Y polarization channel are separated by properly adjusting the polarization control function. Methods to separate orthogonal polarization channels in a polarization multiplexing optical receiver are described in, for example, S. Hinz et al., “Interference detection enabling 2×20 Gbit/s RZ polarization division multiplex transmission”, Electronics Letters, Vol. 37, No. 8, pp. 510-511, April 2001; Yagi et al., “Field Trial of 214-km-long 160-Gbit/s Polarization-Division Multiplexed RZ-DQPSK Transmission System using Automatic Polarization Control” IEICE General Conference, BS-7-9, 2008; and Ito et al., “Comparison of 100 Gbit/s transmission performances between RZ-DQPSK and polarization multiplexed NRZ/RZ-DPSK with automatic polarization de-multiplexer”, OFC/NFOEC 2008, JThA46 (non-patent documents 1-3). Related technologies are also disclosed in Japanese Laid-open Patent Publication No. 05-136761, Japanese Laid-open Patent Publication No. 05-327576, and Japanese Laid-open Patent Publication No. 2002-344426 (patent documents 1-3).
In the conventional technology, the X polarization channel and the Y polarization channel have not necessarily been properly separated. Especially in an environment in which the orthogonality between the X polarization channel and the Y polarization channel is deteriorated by, for example, polarization dependent loss, it has been difficult to ensure the quality of both data signals transmitted by the X polarization channel and the Y polarization channel. In the conventional polarization multiplexing optical transmission system, for example, even if the quality of one received data signal is high, that of the other may sometimes be lower.